The present invention relates to a pressure sensor and, more particularly, to a digital pressure sensor which can compensate for variations of temperature, humidity, a power supply voltage, etc.
A digital pressure sensor is developed, which develops a signal of which frequency varies in response to variations of a detected pressure. Such a digital pressure sensor is applicable to a sphygmomanometer including a digital display. In the conventional digital pressure sensor, the output frequency varies, without regard to the variation of the detected pressure, when temperature, humidity or a power supply voltage varies. This will preclude an accurate detection of the blood pressure.
Accordingly, an object of the present invention is to provide a digital pressure sensor which can compensate for variations of temperature, humidity, a power supply voltage, etc.
Another object of the present invention is to provide a pressure sensor suited for a sphygmomanometer including a digital display.
Other objects and further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. It should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.
To achieve the above objects, pursuant to an embodiment of the present invention, a first oscillator develops a first signal of which frequency represents a detected pressure. A second oscillator of the same construction as the first oscillator develops a reference frequency signal, which functions to gate the first signal for a predetermined period of time. Since the first and second oscillators have the same constructions, the pressure sensor of the present invention can compensate for variations of temperature, humidity and a power supply voltage.
More specifically, the reference frequency signal derived from the second oscillator is applied to a frequency divider which develops a gate signal to be applied to one input terminal of an AND gate, of which the other input terminal is connected to receive the first signal derived from the first oscillator. An output signal of the AND gate is applied to a counter for detecting the pressure.